Insulating suspended timber floors

4 Sep 2019
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United Kingdom
Hello all,

looking for a little advice.

I am insulating two ground floors. Both are suspended timber on 100mm joists. One has an undercroft, the other is over a basement which has a lathe and plaster ceiling.

For the undercroft is there any reason that I cannot fill with 100mm celotex (friction fit and suspended by battens tacked across the joists) given that the ends of the joists will be open to the undercroft (about 4 feet deep)? There are air bricks to the outside, which I will fit with flexible vents to take the air flow below the celotex. and cross flow from air bricks in the adjoining cellar.

For the one with the lathe and plaster ceiling below (which is venitlated) I will presumably need to leave an air gap, but I cannot work out whether that should be 25mm or 50mm.

Would I also be wise to fit a polyethelene barrier across the entire floor joists before relaying the floor?

Thanks in advance.

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How are the ends of the joists open?
Are they not built into the wall?

You shouldn’t need polythene sheeting if ventilation is good, as moisture will be wicked away.

Presume the air vents are lower down than the joist line?
Sorry - i meant the bottom of the joists.

the vents are at the joist line hence why I’m fitting the flexible vents.

the polyethylene barrier was to prevent any potential draughts, although if I’ve fitted it correctly there shouldn’t be any.

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